1. Field of the Invention
The present invention relates to an optical head and a photodetector used in an optical head. More specifically, the present invention relates to an optical head used in an optical information apparatus in which data is recorded, reproduced, and erased by using an optical beam such as a laser, and a photodetector used in such an optical head.
2. Description of the Related Art
An optical head, which is an important element of an optical information apparatus, is a device including means for focusing light on an optical information storage medium and a means for detecting light reflected from the optical information storage medium. FIG. 1 schematically shows a top face (a light detection face) of a photodetector used in a conventional optical head.
The photodetector 16, as shown in FIG. 1, has a plurality of light receiving regions for signal detection on a substrate and a plurality of terminals on the edge of the substrate. The substrate is made of a semiconductor, and the light receiving regions for signal detection are PN-type or PIN-type photodiodes formed in the semiconductor substrate. The predetermined voltage is applied to the photodiodes in a reverse direction. When the photodiodes are exposed to light, a photocurrent flows across the PN junctions of the photodiodes depending on the amount of the received light. The detection of the amount of the photocurrent makes it possible to obtain the amount of the received light. In order to operate the photodetector 16, it is necessary to supply a predetermined potential (voltage), for example, power level potential, reference level potential, ground level potential, and the like to each part of the semiconductor substrate. The power level voltage is supplied to cathode common terminals 17P1 and 17P2 to operate the photodetector 16 shown in FIG. 1.
In an optical head adopting a three-beam method, one of the beams emitted from a light source is diffracted by a diffraction element. Among the diffracted light beams formed by the diffraction element, a zero-order diffracted light beam is used as a main beam, and first-order and minus-first-order diffracted light beams are used as sub-beams for tracking. Among the three light beams formed in the optical head, the main beam forms an optical spot 18 in the middle of signal detection regions 16A, 16B, 16C, and 16D (four divided light receiving regions) of the photodetector 16, after the main beam is reflected by an optical disk. A focus error signal can be obtained from a differential signal between output signals of the diagonal regions of the signal detection regions 16A, 16B, 16C and 16D. More specifically, the focus error signal is obtained by the calculation of a known method based on output signals from anode terminals 17A, 17B, 17C, and 17D connected to the signal detection regions 16A, 16B, 16C, and 16D.
On the other hand, the two sub-beams form two sub-light spots 19A and 19B on the signal detection regions 16E and 16F, after the two sub-beams are reflected by the optical disk. A tracking error signal can be obtained from the differential signal between outputs of signal detection regions 16E and 16F. More specifically, the tracking error signal can be obtained by the calculation of a known method based on output signals from the anode terminals 17E and 17F.
Data signal recorded on the optical disk can be detected from a signal obtained by adding signals of the four divided receiving regions 16A, 16B, 16C, and 16D. More specifically, calculation is performed by a known method based on output signals from the anode terminals 17A, 17B, 17C, and 17D corresponding to the signal detection regions 16A, 16B, 16C, and 16D.
In the conventional photodetector 16 of the optical head as described above, in the case where stray light is generated in the optical head, stray light spots 20A and 20B are often formed in the peripheral area of the signal detection regions of the photodetector. Particularly, in the case of the optical head adopting the three-beam method, stray light frequently occurs. When stray light is irradiated on the peripheral area of the signal detection regions of the photodetector, the correct focus error signal, the tracking error signal, and the data signal cannot be stably obtained for the following reason.
When the stray light is irradiated on the semiconductor substrate of the photodetector 16, carriers (hereinafter, referred to as "stray light carriers") are generated in the semiconductor substrate. Even if the stray light carriers are generated in the region other than the signal detection regions, the stray light carriers diffuse in the semiconductor substrate to reach the vicinity of the signal detection region. When part of the stray light carriers reaches a depletion layer of the signal detection regions, the force from the electric field of the depletion layer forces the stray light carriers to flow into the signal detection region. As a result, a current flows through the photodiode of the signal detection regions. Thus, the current due to the stray light causes unnecessary signals. A case, for example, where the stray light spot 20A is formed at the position indicated in FIG. 1 is herein considered. The stray light carriers that are generated by the stray light spot 20A diffuse in all directions. However, the amount of stray light carriers flowing into the signal detection region 16A which is relatively near is larger than that flowing into the signal detection region 16C which is relatively far. As a result, for example, a correct focus error signal cannot be obtained. As described above, the occurrence of the stray light in the conventional optical head makes the stable detection of signals difficult.